The present invention relates to coherent imaging systems using vibratory energy, such as ultrasound and the like, and, more particularly, to a novel method and apparatus for digital baseband beam formation in which all of a plurality of phased array channels receive a uniform clock signal.
In a recent U.S. Pat. 4,839,652, issued June 13, 1989, a method for phased array beam formation was described. This method uses a plurality N of individual channels each having a digital baseband demodulator which requires a variable clock signal for use with a fixed phase demodulator to perform phased array beam formation with high accuracy. While highly useful, the particular approach of that beam former may experience some practical problems in implementation, as an independent variable-clock signal must be provided for each of the multiplicity of phased array channels. That is, in order to form beams of acceptable quality for real-time phased-array vibratory energy (ultrasound) imaging, a coherent sum across all of the channels of the phased array must be performed with great accuracy. The primary source of coherent summation error is the quantization of the time delay t.sub.d,j in each j-th channel (where 1.ltoreq.j.ltoreq.N). In the method of the above-mentioned patent, accurate time delays are provided by deliberately slipping a master clock signal, utilized to synchronize sampling in the analog-to-digital converters (ADC) of each channel, in time intervals small compared to the sampling period of the clock signal itself. It is therefore highly desirable to provide a method and apparatus utilizing a single fixed clock, with common timing provided to each of the array channels, and capable of producing the same quality images as the time-slipped variable clock implementation of the aforementioned patent.